Methods of making single-layer lithium ion battery separators having nanofiber and microfiber components

ABSTRACT

An insulating (nonconductive) microporous polymeric battery separator comprised of a single layer of enmeshed microfibers and nanofibers is provided. Such a separator accords the ability to attune the porosity and pore size to any desired level through a single nonwoven fabric. Through a proper selection of materials as well as production processes, the resultant battery separator exhibits isotropic strengths, low shrinkage, high wettability levels, and pore sizes related directly to layer thickness. The overall production method is highly efficient and yields a combination of polymeric nanofibers within a polymeric microfiber matrix and/or onto such a substrate through high shear processing that is cost effective as well. The separator, a battery including such a separator, the method of manufacturing such a separator, and the method of utilizing such a separator within a battery device, are all encompassed within this invention.

FIELD OF THE INVENTION

The present invention relates to an insulating (nonconductive)microporous polymeric battery separator comprised of a single layer ofenmeshed microfibers and nanofibers. Such a separator accords theability to attune the porosity and pore size to any desired levelthrough a single nonwoven fabric. Through a proper selection ofmaterials as well as production processes, the resultant batteryseparator exhibits isotropic strengths, low shrinkage, high wettabilitylevels, and pore sizes related directly to layer thickness. The overallproduction method is highly efficient and yields a combination ofpolymeric nanofibers within a polymeric microfiber matrix and/or ontosuch a substrate through high shear processing that is cost effective aswell. The separator, a battery including such a separator, the method ofmanufacturing such a separator, and the method of utilizing such aseparator within a battery device, are all encompassed within thisinvention.

BACKGROUND OF THE INVENTION

Batteries have been utilized for many years as electrical powergenerators in remote locations. Through the controlled movement of ionsbetween electrodes (anode and cathode), a power circuit is generated,thereby providing a source of electricity that can be utilized until theexcess ions in one electrode are depleted and no further electricalgeneration is possible. In more recent years, rechargeable batterieshave been created to allow for longer lifetimes for such remote powersources, albeit through the need for connecting such batteries to otherelectrical sources for a certain period of time. All in all, however,the capability of reusing such a battery has led to greater potentialsfor use, particularly through cell phone and laptop computer usage and,even more so, to the possibility of automobiles that solely requireelectricity to function.

Such batteries typically include at least five distinct components. Acase (or container) houses everything in a secure and reliable manner toprevent leakage to the outside as well as environmental exposure inside.Within the case are an anode and a cathode, separated effectively by aseparator, as well as an electrolyte solution (low viscosity liquid)that transport ions through the separator between the anode and cathode.The rechargeable batteries of today and, presumably tomorrow, will runthe gamut of rather small and portable devices, but with a great deal ofelectrical generation potential in order to remain effective for longperiods between charging episodes, to very large types present withinautomobiles, as an example, that include large electrodes (at least insurface area) that must not contact one another and a large number ofions that must consistently and constantly pass through a membrane tocomplete the necessary circuit, all at a level of power generationconducive to providing sufficient electricity to run an automobilemotor. As such, the capability and versatility of battery separators inthe future must meet certain requirements that have yet to be providedwithin the current industry.

Generally speaking, battery separators have been utilized since theadvent of closed-cell batteries to provide necessary protection fromunwanted contact between electrodes as well as to permit effectivetransport of electrolytes within power generating cells. Typically, suchmaterials have been of film structure, sufficiently thin to reduce theweight and volume of a battery device while imparting the necessaryproperties noted above at the same time. Such separators must exhibitother characteristics, as well, to allow for proper battery function.These include chemical stability, suitable porosity of ionic species,effective pore size for electrolyte transfer, proper permeability,effective mechanical strength, and the capability of retainingdimensional and functional stability when exposed to high temperatures(as well as the potential for shutdown if the temperature rises to anabnormally high level).

In greater detail, then, the separator material must be of sufficientstrength and constitution to withstand a number of different scenarios.Initially, the separator must not suffer tears or punctures during thestresses of battery assembly. In this manner, the overall mechanicalstrength of the separator is extremely important, particularly as hightensile strength material in both the machine and cross (i.e.,transverse) directions allows the manufacturer to handle such aseparator more easily and without stringent guidelines lest theseparator suffer structural failure or loss during such a criticalprocedure. Additionally, from a chemical perspective, the separator mustwithstand the oxidative and reductive environment within the batteryitself, particularly when fully charged. Any failure during use,specifically in terms of structural integrity permitting abnormally highamounts of current to pass or for the electrodes to touch, would destroythe power generation capability and render the battery totallyineffective. Thus, even above the ability to weather chemical exposure,such a separator must also not lose dimensional stability (i.e., warp ormelt) or mechanical strength during storage, manufacture, and use,either, for the same reasons noted above.

Simultaneously, however, the separator must be of proper thickness to,in essence, facilitate the high energy and power densities of thebattery, itself. A uniform thickness is quite important, too, in orderto allow for a long life cycle as any uneven wear on the separator willbe the weak link in terms of proper electrolyte passage, as well aselectrode contact prevention.

Additionally, such a separator must exhibit proper porosity and poresizes to accord, again, the proper transport of ions through such amembrane (as well as proper capacity to retain a certain amount ofliquid electrolyte to facilitate such ion transfer during use). Thepores themselves should be sufficiently small to prevent electrodecomponents from entering and/or passing through the membrane, while alsoallowing, again, as noted above, for the proper rate of transfer ofelectrolyte ions. As well, uniformity in pore sizes, as well as poresize distribution, provides a more uniform result in power generationover time as well as more reliable long-term stability for the overallbattery as, as discussed previously, uniform wear on the batteryseparator, at least as best controlled in such a system, allows forlonger life-cycles. It additionally can be advantageous to ensure thepores therein may properly close upon exposure to abnormally hightemperatures to prevent excessive and undesirable ion transfer upon sucha battery failure (i.e., to prevent fires and other like hazards).

As well, the pore sizes and distributions may increase or decrease theair resistance of the separator, thus allowing for simple measurementsof the separator that indicate the ability of the separator to allowadequate passage of the electrolyte present within the battery itself.For instance, mean flow pore size can be measured according to ASTME-1294, and this measurement can be used to help determine the barrierproperties of the separator. Thus, with low pore size, the rigidity ofthe pores themselves (i.e., the ability of the pores to remain a certainsize during use over time and upon exposure to a set pressure) allowsfor effective control of electrode separation as well. More importantly,perhaps, is the capability of such pore size levels to limit dendriteformation in order to reduce the chances of crystal formation on ananode (such a lithium crystals on a graphite anode) that woulddeleteriously impact the power generation capability of the battery overtime.

Furthermore, the separator must not impair the ability of theelectrolyte to completely fill the entire cell during manufacture,storage and use. Thus, the separator must exhibit proper wicking and/orwettability during such phases in order to ensure the electrolyte infact may properly transfer ions through the membrane; if the separatorwere not conducive to such a situation, then the electrolyte would notproperly reside on and in the separator pores and the necessary iontransmission would not readily occur. Additionally, it is understoodthat such proper wettability of the separator is generally required inorder to ensure liquid electrolyte dispersion on the separator surfaceand within the cell itself. Non-uniformity of electrolyte dispersion mayresult in dendritic formations within the cell and on the separatorsurface, thereby creating an elevated potential for battery failures andshort circuiting therein.

There is also great concern with the dimensional stability of such aseparator when utilized within a typical lithium ion cell, as alluded toabove. The separator necessarily provides a porous barrier for iondiffusion over the life of the battery, certainly. However, in certainsituations, elevated temperatures, either from external sources orwithin the cell itself, may expose susceptible separator materials toundesirable shrinking, warping, or melting, any of which maydeleteriously affect the capability of the battery over time. As such,since reduction of temperature levels and/or removal of such batterytypes from elevated temperatures during actual utilization are verydifficult to achieve, the separator itself should include materials thatcan withstand such high temperatures without exhibiting any appreciableeffects upon exposure. Alternatively, the utilization of combinations ofmaterials wherein one type of fiber, for instance, may provide such abeneficial result while still permitting the separator to perform at itsoptimum level, would be highly attractive.

To date, however, as noted above, the standards in place today do notcomport to such critical considerations. The general aim of an effectivebattery separator is to provide such beneficial characteristics allwithin a single thin sheet of material. The capability to provide lowair resistance, very low pore size and suitable pore size distribution,dimensional stability under chemical and elevated temperatureenvironments, proper wettability, optimal thickness to permit maximumbattery component presence in the smallest enclosure possible, andeffective overall tensile strength (and preferably isotropic in nature),are all necessary in order to accord a material that drastically reducesany potential for electrode contact, but with the capability ofcontrolled electrolyte transport from one portion of the battery cell tothe other (i.e., closing the circuit to generate the needed electricalpower), in other words for maximum battery output over the longestperiod of time with the least amount of cell volume. Currently, suchproperties are not effectively provided in tandem to such a degree. Forinstance, Celgard has disclosed and marketed an expanded film batteryseparator with very low pore size, which is very good in that respect,as noted above; however, the corresponding air resistance for such amaterial is extremely high, thus limiting the overall effectiveness ofsuch a separator. To the contrary, duPont commercializes a nanofibernonwoven membrane separator that provides very low air resistance, butwith overly large pore sizes therein. Additionally, the overallmechanical strengths exhibiting by these two materials are verylimiting; the Celgard separator has excellent strength in the machinedirection, but nearly zero strength in the cross (transverse) direction.Such low cross direction strength requires very delicate handling duringmanufacture, at least, as alluded to above. The duPont materials fare alittle better, except that the strengths are rather low in bothdirections, albeit with a cross direction that is higher than theCelgard material. In actuality, the duPont product is closer to anisotropic material (nearly the same strengths in both machine and crossdirections), thus providing a more reliable material in terms ofhandling than the Celgard type. However, the measured tensile strengthsof the duPont separator are quite low in effect, thus relegating theuser to carefully maneuvering and placing such materials duringmanufacture as well. Likewise, the dimensional stability of such priorbattery separators are highly suspect due to these tensile strengthissues, potentially leading to materials that undesirably lose theirstructural integrity over time when present within a rechargeablebattery cell.

Thus, there still exists a need to provide a battery separator thatsimultaneously provides all of these characteristics for long-term,reliable, lithium battery results. As such, although such a separatorexhibiting low air resistance and low pore size, as well as high tensilestrength overall and at relatively isotropic levels, proper chemicalstability, structural integrity, and dimensional stability (particularlyupon exposure to elevated temperatures), although highly desired, todate there has been a decided lack of provision of such a prizedseparator material. Additionally, a manner of producing batteryseparators that allows for achieving such desired targeted propertylevels through efficient manufacturing processes would also be highlydesired, particularly if minor modifications in materials selection,etc., garners such beneficial results and requirements on demand;currently, such a manufacturing method to such an extent has yet to beexplored throughout the battery separator industry. As such, aneffective and rather simple and straightforward battery separatormanufacturing method in terms of providing any number of membranesexhibiting such versatile end results (i.e., targeted porosity and airresistance levels through processing modifications on demand) as well asnecessary levels of mechanical properties, heat resistance,permeability, dimensional stability, shutdown properties, and meltdownproperties, is prized within the rechargeable battery separatorindustry; to date, such a material has been unavailable.

ADVANTAGES AND SUMMARY OF THE INVENTION

A distinct advantage of the present invention is the ease inmanufacturing through a wet-laid nonwoven fabrication process. Anotherdistinct advantage is the resulting capability of providing any targetedlevel of pore size, porosity, and air resistance, through the merechange in proportions of component fibers utilized during thefabrication process, as well as the proper calendering of the producedsingle layer material. Yet another advantage of this inventive batteryseparator is the isotropic strength properties accorded the user forreliability in long-term use as well as during the battery manufacturingphase. The ability of the inventive separator to provide contemporaneouslow air resistance and low pore sizes is still a further advantage ofthis invention. Yet another advantage of this inventive batteryseparator is the provision of a specifically non-conductive (and thusinsulating) fabric (or paper) that does not allow transmission ofelectrical charge through the separator body, but solely through thetransport of charged ions through the pores present within itsstructure. Yet another advantage is the high porosity of the material,allowing more ions to flow and increasing the durability of the abilityto hold energy over many life cycles by allowing fully rechargedelectrodes. Other advantages include, without limitation, the ability todial in certain physical characteristics through the proper selection offibrous materials prior to layer formation, as well as the utilizationof all micro fibers initially and the generation of nanofibers (infibrillated form) through high shear treatment thereof and thus thecapability of forming all the necessary separator components from asingle starting material.

Accordingly, this invention pertains to a method of forming a batteryseparator, wherein said battery separator exhibits a maximum thicknessof 250 microns, and wherein said battery separator includes acombination of microfiber and nanofiber constituents, said methodcomprising the steps of: a) providing an aqueous solvent; b) introducingtherein a plurality of nanofibers to form a nanofiber dispersion withinan aqueous solvent; c) mixing said nanofiber dispersion under high shearconditions; d) introducing a plurality of microfibers to form amicrofiber/nanofiber dispersion within an aqueous solvent; e)introducing said highly sheared dispersion within a paper makingmachine; f) producing a web of microfiber/nanofiber material; and g)drying said web. The invention further encompasses such a method whereinsaid resultant web of step “f” is further treated in a calenderingprocedure to produce a separator material exhibiting a thickness of atmost 100 microns and a pore size of at most 2000 nm.

The resultant polymeric battery separator made from this inventivemethod thus comprises a nonwoven combination of microfibers andnanofibers, wherein said single layer of said separator exhibits anisotropic tensile strength with the machine direction tensile strengthless than three times the cross direction tensile strength. Saidseparator also exhibits a machine direction (MD) tensile strengthgreater than 90 kg/cm² and less than 1,000 kg/cm², a cross direction(CD) tensile strength greater than 30 kg/cm² and less than 1,000 kg/cm²,and a mean flow pore size less than 0.80 μm. Furthermore, such aninvention includes a battery separator as defined and comprising asingle layer of fibers, said layer comprising both nanofibers andmicrofibers, said nanofibers having an average maximum width less than1000 nm, said microfibers having a maximum width greater than 3000nanometers, and said nanofibers and microfibers intermingled such thatat least a portion of said nanofibers reside in the interstices betweensaid microfibers.

Throughout this disclosure, the term microfiber is intended to mean anypolymeric fiber exhibiting a width that is measured in micrometers,generally having a maximum width greater than 1000 nm, but also greaterthan 3000 nm, or even greater than 5000 nm or possibly even greater than10,000 nm, up to about 40 microns. As well, the term nanofiber isintended to mean any polymeric fiber exhibiting a width that is measuredin nanometers, generally having a maximum width less than 1000 nm, butpossibly less than 700 nm, or even less than 500 nm or possibly evenless than 300 nm (as low as about 1 nm). For either micro fiber ornanofiber materials, it should be understood that width may beconsidered diameter, although in such a situation, diameter would beconsidered a mean diameter since uniformity of fiber structure istypically very difficult to achieve. Thus, maximum width is utilized asthe primary definition, particularly if the fibers themselves are notcylindrical in shape, thus allowing for the possibility of square,rectangle, triangle, or other geometric shape(s) for such fibers, whichwould all be within the scope of breadth of this invention as long asthe proper micro- and nano-fiber measurements are present. As well, theterm insulating in intended to indicate no appreciable degree ofelectrical conductivity, and thus the inventive fabric structure doesnot permit electrical charge throughout the fabric body, but onlythrough the passage of electrolytic ions through the pores presenttherein.

Such a combination of microfibers and nanofibers has yet to beinvestigated within the battery separator art, particularly in terms ofthe capability of providing a single-layer nonwoven fabric of the twobase components for such a purpose. The combination is particularlyimportant, as it provides a nonwoven with a bimodal distribution offiber diameters and lengths, such that the average length of themicrofibers is at least 5 times the average length of the nanofibers,preferably longer than 10 times the average length of the nanofibers,and most preferably longer than 20 times the length of the nanofibers.Additionally, the diameters are also in a bimodal distribution, suchthat the average diameter of the microfibers is more than 3 times theaverage diameter of the nanofibers, preferably more than 5 times theaverage diameter of the nanofibers, and most preferably greater than 10times the average diameter of the nanofibers. This bimodal distributionallows the microfibers to provide strength, loft, permeability, modulus,tear and puncture resistance, wet strength, processability, and otherfeatures that the nanofibers could not provide on their own.

The microfiber constituent may be of any suitable polymer that providesthe necessary chemical and heat resistance alluded to above, as well asthe capability of forming a microfiber structure. As well, such amicrofiber may also be fibrillated (or treated in any other like manner,such as through plasma exposure, and the like) during or subsequent tofiber formation in order to increase the surface area thereof tofacilitate the desired entangling between a plurality of suchmicrofibers during a nonwoven fabrication process. Such polymericcomponents may thus include acrylics such as polyacrylonitrile,polyolefins such as polypropylene, polyethylene, polybutylene and othersincluding copolymers, polyamides, polyvinyl alcohol, polyethyleneterephthalate, polybutylene terephthalate, polysulfone, polyvinylfluoride, polyvinylidene fluoride, polyvinylidenefluoride-hexafluoropropylene, polymethyl pentene, polyphenylene sulfide,polyacetyl, polyurethane, aromatic polyamide, semi-aromatic polyamide,polypropylene terephthalate, polymethyl methacrylate, polystyrene,cellulosic polymers (rayon, as one non-limiting example), polyaramids,including para-aramids and meta-aramids, and blends, mixtures andcopolymers including these polymers. Polyacrylates, cellulosic polymers,and polyaramids are potentially preferred. Such materials provide arange of highly desirable properties that function in combination withthe properties of the other polymer types to accord overarchingbeneficial results in terms of tensile strength, high temperatureprotection, wettability, and pore size capability, particularly whenincluded as nanofiber components with like microfiber bases. Suchmicrofibers may also be pre-treated with adhesives to effectuate thedesired degree of contact and dimensional stability of the overallnonwoven structure subsequent to fabrication.

Additionally, the microfibers may be selected in terms of individualfiber properties to provide combinations of materials that accorddesirable characteristics to the overall battery separator. Thus, sincepoly-aramid, meta-aramid, and cellulosic fibers provide excellent heatresistance and certain strength benefits, such fibers may beincorporated individually (as wet-laid constituents, for example) or incombination through entanglement or other means. Such fibers must be ofsufficient length to impart the necessary strength to the overallseparator but short enough to permit proper incorporation (such as,again, for instance, within a wet-laid procedure). For instance, theymay preferably be longer than 0.5 mm, more preferably longer than 1 mm,and most preferably longer than 2 mm.

Microfibers or nanofibers may preferentially be of a material that willmelt or flow under pressure or high temperature. It is of particularbenefit to have one constituent which will melt or flow at a temperaturethat is lower than the other constituents. For example, polyestermicrofibers can be made to flow at temperatures approaching the melttemperature of 260° C. Additionally, polyacrylonitrile microfibers ornanofibers can be made to flow under high pressure and temperature.Cellulose, rayon, aramid, and other micro- or nanofibers will not flowunder these temperatures. Thus, a combination of materials comprising atleast one fiber that will flow under high temperature and/or pressureand at least one fiber that will not flow under the same temperatureand/or pressure will enable the first fiber to bond the other fiberstogether, imparting additional strength to the nonwoven separator.

The nanofibers may thus be of any like polymer constituency and/orcombination in order to withstand the same types of chemical and hightemperature exposures as for the microfibers. Due to their size, thereis no requirement of post-manufacture treatment of such nanofibermaterials to accord any increase in entanglement on the producednonwoven surface or within the interstices thereof. Nanofibergeneration, however, may be provided through a high shear treatment ofmicrofiber sources in order to strip nanosized constituents there fromas materials that meet the definition of nanofiber, above. In thismanner, too, such peeled or stripped nanofibers will exhibit afibrillated appearance and performance such that improvements inentanglement within the interstices of the microfibers during separatorproduction may occur (not to mention the potential for improvedentanglement between individual nanofibers prior to and/or during thesame separator manufacturing procedure). In such a situation, themicrofiber and nanofiber materials may thus be from the same material,with portions of the microfiber material removed to form the nanofiberconstituents, and such nanofibers may have varying and multiple lengthsas well as varied cross sections and overall sizes. In any event,nanofiber production may be undertaken in this manner with the removedconstituents from the microfiber source collected and utilized in suchforms with other types of microfibers, not just those from which suchnanofibers have been provided. In such embodiments of the inventivebattery separator, any type of nanofiber may be utilized for such apurpose. Preferably, however, the capability of providing nanofibersthat exhibit potentially beneficial properties, such as high temperaturestability, tensile strength, and the like, may create a situationwherein specific fiber types are utilized.

Although such “fibrillated” nanofibers may be utilized are described,specifically produced nanofiber components may be incorporated with suchmicrofiber base materials to permit the inventive result of a separatorsheet with certain pore sizes produced through a wet-laid process. Sucha manufacturing process thus includes the introduction of nanofibercomponents within a microfiber solution in a dilute wet state, mixingthe same under high shear conditions, and then drying to form aresultant sheet. This sheet may then be calendered in order to reducethe sheet thickness as desired, but, in addition, to further dial in theoptimal pore sizes and pore size distribution present therein. With aresilient sheet of properly dispersed and incorporated microfiber andnanofiber components, this wet-laid process permits suitable sheetproduction wherein the amount of nanofiber dictates the capacity to fillthe interstices between microfiber constituents, thus creating thedesired pores within the resultant sheet. The calendering operation maythen permit a correlative value of sheet thickness to pore size,particularly due to the overall tensile strength of the sheet subsequentto wet-laid production. Such a process thus provides a relativelysimple, yet elegant method to provide the capability of optimizing poresize distribution and size without having to extrude or otherwisemanipulate the overall structure in a manner that may deleteriouslytear, warp, and/or obfuscate the dimensional stability thereof.Additionally, the ability to utilize a simple microfiber/nanofiber/watersolution for the sheet production process allows for, again, an elegantand simple method, but also one that reduces or even eliminates the needfor other chemicals to impart the desired production scheme. Such a purestarting material and overall production method further underscores thehighly unexpected benefits of not only the method employed for such aninventive product, but the simple combination of microfibers withnanofibers and an aqueous solute for such a purpose and yet to achieve aheretofore unattainable battery separator material on demand and withthe versatility for multiple end uses.

Thus, it is of great importance for the inventive method and productsthat the nanofiber constitutents combine with the microfibers under asufficiently high shear environment to accord the desired introductionof such nanofibers onto and within the resultant microfiber nonwovensubstrate simultaneously during actual nonwoven fabrication itself. Inother words, upon the provision of both types of fiber materials withinthe nonwoven production process, the manufacturer should accord asufficient amount of mixing and high shear conditions to best ensure theproper degree of entanglement between the different fiber types to formthe desired single-layer fabric structure. As well, the fabricationmethod is potentially preferred as a wet-laid nonwoven procedure inaddition to the high shear type, ostensibly to best ensure the properintroduction and residual location of nanofibers within the microfiberinterstices. With an increased water flow during manufacture, theextremely small nanofibers will be drawn into such interstices at agreater rate than with a dry entanglement method, thereby according theaforementioned interstice fill capability. Again, the higher the waterlevel for such a purpose, the greater purity (and recovery of water andexcess fibers, for that matter, for further utilization in a separatebattery separator manufacturing process) and reliability for suitablenanofiber entanglement within the microfiber base. The resultantnonwoven structure would thus exhibit greater uniformity in terms ofthickness, porosity, and, most importantly, pore sizes, therein, as wellas more reliable stability for calendering to optimize thickness andpore size results, as noted above.

One method followed for such a wet-laid procedure includes the provisionof pre-fibrillated microfibers in a pulp-like formulation, comprising,for example, from 50:1 to 10000:1 parts water per part of fiber (again,water alone is preferred, although, if desired, other solvents thatpermit a wet-laid process and subsequent facilitation of evaporationthereof may be utilized, including, for instance, certain non-polaralcohols). The pre-fibrillated microfibers have been treated in such amanner, thus exhibiting a certain amount of already-present nanofibers(the residual product removed from the microfiber themselves duringfibrillation, but not removed from the overall resultant mesh ofmicrofibers thereafter). Such pre-fibrillated microfibers and nanofibersare in pulp form as a result of the fibrillation procedure, rendering aslurry-like formulation including the above-noted aqueous-based solventwith the resultant pre-fibrillated microfibers and nanofibers. Thisslurry-like formulation is then mixed with selected amounts of othermicrofibers and/or nanofibers (preferably in pulp- or slurry-like form,as well), or the initial slurry is mixed alone, and the resultantformulation can be heated in hot water to a temperature of at least 60°C., more preferably at least 70, and most preferably at least 80, havinga very low concentration of actual fiber solids content therein (i.e.,below 1% and as low as less than 0.5% or even less than 0.1% by weightof water or other aqueous-based solvent). This heated dispersion is thensubjected to a high shear environment with subsequent placement on aflat surface. Such a surface is sufficiently porous to allow for solventelution, thus leaving the desired wet-laid nonwoven single fabric layerincluding fibrillated microfibers entangled with one another, andexhibiting interstices between each microfiber, as well as nanofiberspresent within such interstices and on the surface of the largermicrofibers as well. The amount of added nanofibers to thepre-fibrillated microfiber pulp would thus accord greater amounts offill between the microfiber interstices to provide an overall low meanpore size, particularly in comparison to a wetlaid nonwoven that is madesolely from the pre-fibrillated pulp alone. Conversely, then, theaddition of microfibers to the pre-fibrillated fiber slurry would accorda larger mean pore size to the resultant wetlaid nonwoven single layerfabric than the pre-fibrillated fiber slurry alone. This capability totarget different mean pore sizes through nanofiber and/or microfiberaddition levels accords the manufacturer the potential to achieve anydesired mean pore size level.

Subsequent to such a high-shear mixing step, the resultant dispersionmay be fed into the head of a paper machine (of any type that is capableof making light weight sheets without breaking, such as, as merelyexamples, Fourdrinier, Incline Wire, Rotoformer, and the like, devices).Such light weight sheets may be produced through controlling the fiberdispersion input in the head end with simultaneously controlled linespeed. A set-up wherein no open draws are present (i.e., wherein the wetfiber web is unsupported) is preferred for such a method. In thissituation, the high water level may be alleviated through vacuum means(which is a common step in the paper making industry), at leastinitially (i.e., to remove surface moisture to a certain level). For theproper thin sheet result, a fine gauge paper making wire is necessary,particularly at a gauge of at most 40 gauge, more preferably at most 80gauge. The paper (dispersion sheet) width may be accorded anymeasurement as long as the production speed does not affect the endresult and the overall tensile strength (particularly in an isotropicfashion) is not compromised. For efficiency purposes, the line speed maybe set within a range of 25 to 1,500 ft/min, more preferably with aminimum of 50, and most preferably 100.

After such a paper (sheet) making step is accomplished, the formed sheetmay be introduced within a drying device. Any type of standard dryingmeans may be utilized, including heated steam cans or a hot air oven.Such heating should exceed the temperature necessary to evaporate thewater (or other solvents), but should not be so high as to melt ordeform the sheet itself. Such drying temperatures thus may depend uponthe materials in use, as well as the sheet thicknesses, as certainmaterials may withstand higher temperatures than others in terms ofdimensional stability and the thicker the sheet, typically the greatertemperature resistance to warping or other effect.

The manufacturer may thus control the desired properties of theinventive battery separators through the capability of providingdifferent thicknesses of the single-layer structure on demand as well.Such a thickness characteristic may be provided through the initialwet-laid fabrication method process parameters alone, or themanufacturer may subsequently calendar the resultant fabric to anydesired thickness. The potential to calendar and otherwise alter thethickness of the resultant single layer fabric permits the manufacturerthe further capability to allow for greater versatility in terms of bothair resistance and mean pore size measurements. Such a dial-in processhas yet to be explored within the battery separator industry. Acalendering step utilizing typical devices, such as hard steel rolls, ora combination of a single hard steel roll and a second hard rubber roll,as merely examples, may be employed. The calendaring step maypreferentially be heated to a temperature above 200° F., preferentiallyabove 250, or even above 300. Multiple calendering steps may beundertaken as well for such a purpose, if the materials can withstandsuch activities without any appreciable loss of tensile strength, etc.,as noted above, as well.

Resultant thicknesses may thus be less than 250 micrometers, preferablyless than 100 micrometers, more preferably less than 50 micrometers,even more preferably less than 35 micrometers, most preferably less than25 micrometers. The areal density of the sheets are also of importance,and these methods allow the achievement of light sheets which are usefulfor battery separators especially to create small, light weightbatteries. As such, sheet weights below 30 grams/m² are desirable, evenbelow 20 grams/m², or even 15 grams/m². As noted above, the capabilityof preventing contact between the anode and cathode of the battery isnecessary to prevent a shorted circuit during battery use; the thicknessof the separator and the controlled pore size therein provide theessential manner of achieving such a result. However, battery separatorthickness may also contribute to the available volume of other componentparts within the closed battery cell as well as the amount ofelectrolyte solution provided therein. The entirety of the circumstancesinvolved thus require an effective separator in terms of multiplevariables. The beneficial ease of manufacture as well as the capabilityof providing effective on-demand pore size and air resistance propertiesthrough the inventive manufacturing method and the resultantsingle-layer battery separator made therefrom thus sets this developmentdistinctly apart from the state of the art battery separators currentlyused and marketed today.

Other methods of nonwoven sheet manufacture which enable theentanglement of a combination of nanofibers and microfibers may also beused to create the inventive battery separators. One method would be tostart with distinct nanofibers and microfibers and combine them in themethod described above. Other such methods include carding, crosslapping, hydroentangling, air laid, needlepunch, melt blown, spunbond orother methods or combinations of methods that enable the microfibers toform an entangled mesh and the nanofibers to fill the intersticesbetween said microfibers.

In effect, as noted above, the microfiber interstices form the “pores”per se, and the nanofibers fill in such openings to reduce the sizestherein, and to a substantially uniform degree over the entire nonwovenstructure. Of highly unexpected benefit to the overall invention,particularly in terms of targeting different levels of porosity ondemand, is the ability to dial in pore sizes within the resultantnonwoven structure through the mere modification of the concentration ofmicrofibers to nanofibers alone. Thus, for example, a 30% microfiber to70% nanofiber proportion at the nonwoven fabrication process outsetwould provide a pore size in the range of 700 nm to 195 nm, whereas a10% microfiber/90% nanofiber combination would provide an effectivelysmaller pore size distribution (as well as a more uniform range thereof,for example 230 nm to 130 nm). Such an unforeseen result thus accords anon-demand porosity result for the end user through, as noted, as rathersimple manufacturing modification. Such pore sizes created can bemeasured, resulting in a mean flow pore size. Such mean flow pore sizesmay be less than 2000 nm, even less than 1000 nm, preferably less than700 nm, more preferably less than 500 nm.

Additionally, it should be noted that although a single-layer separatorincluding microfibers and nanofibers together is encompassed within thisinvention, the utilization of multiple layers of such a fabricstructure, or of a single layer of such an inventive battery separatorfabric with at least one other layer of a different type of fabric, maybe employed and still within the scope of the overall inventiondescribed herein.

Such battery separators as described herein are clearly useful forimproving the art of primary and rechargeable batteries, but also may beused for other forms of electrolyte conducting energy storagetechniques, such as capacitors, supercapacitors and ultracapacitors.Indeed, the control allowed on the pore size for such inventiveseparators may allow significant improvements in the energy loss, powerdischarge rate, and other properties of these devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM microphotograph of a prior art expanded film batteryseparator.

FIG. 2 is an SEM microphotograph of a prior art nanofiber nonwovenfabric battery separator.

FIGS. 3 and 4 are SEM microphotographs at 1000 and 2000 magnificationlevels of one potentially preferred embodiment of an inventivemicrofiber/nanofiber nonwoven fabric battery separator structure.

FIGS. 5 and 6 are SEM microphotographs at 5000 and 10000 magnificationlevels of another potentially preferred embodiment of an inventivemicrofiber/nanofiber nonwoven fabric battery separator structure.

FIG. 7 shows an exploded view of an inventive rechargeable lithium ionbattery including an inventive battery separator.

DETAILED DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS

All the features of this invention and its preferred embodiments will bedescribed in full detail in connection with the following illustrative,but not limiting, drawings and examples.

Microfiber and Nanofiber Production

As noted above, the microfiber may be constructed from any polymer (orpolymer blend) that accords suitable chemical and heat resistance inconjunction with internal battery cell conditions, as well as thecapability to form suitable fiber structures within the rangesindicated. Such fibers may further have the potential to be treatedthrough a fibrillation or like technique to increase the surface area ofthe fibers themselves for entanglement facilitation during nonwovenfabrication. Such fibers may be made from longstanding fibermanufacturing methods such as melt spinning, wet spinning, solutionspinning, melt blowing and others. In addition, such fibers may begin asbicomponent fibers and have their size and/or shape reduced or changedthrough further processing, such as splittable pie fibers,islands-in-the-sea fibers and others. Such fibers may be cut to anappropriate length for further processing, such lengths may be less than50 mm, or less than 25 mm, or less than 12 mm even. Such fibers may bealso be made long to impart superior processing or higher strength tohave a length that is longer than 0.5 mm, longer than 1 mm, or evenlonger than 2 mm. Such fibers may also be fibrillated into smallerfibers or fibers that advantageously form wet-laid nonwoven fabrics.

Nanofibers for use in the current invention may be made through severallongstanding techniques, such as islands-in-the-sea, centrifugalspinning, electrospinning, film or fiber fibrillation, and the like.Teijin and Hills both market potentially preferred islands-in-the-seananofibers (Teijin's is marketed as NanoFront fiber polyethyleneterephthalate fibers with a diameter of 500 to 700 nm). Dienes andFiberRio are both marketing equipment which would provide nanofibersusing the centrifugal spinning technique. Xanofi is marketing fibers andequipment to make them using a high shear liquid dispersion technique.Poly-aramids are produced by duPont in nanofiber state that exhibitexcellent high temperature resistance, as well as other particularlypreferred properties.

Electrospinning nanofiber production is practiced by duPont, E-SpinTechnologies, or on equipment marketed for this purpose by Elmarco.Nanofibers fibrillated from films are disclosed in U.S. Pat. Nos.6,110,588, 6,432,347 and 6,432,532, which are incorporated herein intheir entirety by reference. Nanofibers fibrillated from other fibersmay be done so under high shear, abrasive treatment. Nanofibers madefrom fibrillated cellulose and acrylic fibers are marketed by EngineeredFiber Technologies under the brand name EFTEC™. Any such nanofibers mayalso be further processed through cutting and high shear slurryprocessing to separate the fibers an enable them for wet laid nonwovenprocessing. Such high shear processing may or may not occur in thepresence of the required microfibers.

Nanofibers that are made from fibrillation in general have a transverseaspect ratio that is different from those made initially as nanofibersin typical fashion (islands-in-the-sea, for instance). One suchtransverse aspect ratio is described in full in U.S. Pat. No. 6,110,588,which is incorporated herein by reference. As such, in one preferredembodiment, the nanofibers have a transverse aspect ratio of greaterthan 1.5:1, preferably greater than 3.0:1, more preferably greater than5.0:1.

As such, acrylic, polyester, and polyolefin fibers are particularlypreferred for such a purpose, with fibrillated acrylic fibers,potentially most preferred. Again, however, this is provided solely asan indication of a potentially preferred type of polymer for thispurpose and is not intended to limit the scope of possible polymericmaterials or polymeric blends for such a purpose.

FIGS. 1 and 2 provide photomicrographs of the typical structures of theCelgard expanded film materials and the duPont nanofiber nonwovenbattery separator materials, respectively, and as discussed above.Noticeably, the film structure of the Celgard separator shows similarityin pore sizes, all apparently formed through film extrusion andresultant surface disruptions in a rather uniform format. The duPontseparator is made strictly from nanofibers alone as the uniformity infiber size and diameter is evident. Being a nonwoven structure of suchnanofibers themselves, the overall tensile strengths of this separatorin both machine and cross directions are very low, although roughlyuniform in both directions. Thus, such a material may be handleduniformly, as a result, although overall strength lends itself to otherdifficulties a manufacturer must face, ultimately, if introducing such aseparator into a battery cell. To the contrary, then, the FIG. 1separator, showing the striations for pore generation in the samedirection (and thus extrusion of the film in one direction), providesextremely high machine direction tensile strength; unfortunately, thetensile strength of the same material in the cross direction is verylow, leaving, as discussed previously, a very difficult and highlysuspect battery separator material to actually utilize in a batterymanufacturing setting.

The inventive materials, shown in photomicrograph form in FIGS. 3 and 4,are of totally different structure from these two prior art products(and are based on Example 39, below). One potentially preferredembodiment of the initial combination of microfiber and nanofibers isthe EFTEC™ A-010-4 fibrillated polyacrylonitrile fibers, which have highpopulations of nanofibers as well as residual microfibers. The resultantnanofibers present within such a combination are a result of thefibrillation of the initial microfibers. Nonwoven sheets made of thesematerials are shown in FIGS. 3 and 4. By way of example, these fiberscan be used as a base material, to which can be added furthermicrofibers or further nanofibers as a way of controlling the pore sizeand other properties of the nonwoven fabric, or such a material may beutilized as the nonwoven fabric battery separator itself. Examples ofsuch sheets with additional microfibers added are shown in FIGS. 5, 6and 7. Typical properties of the acrylic Micro/Nanofibers are shownbelow.

TABLE 1 Acrylic Micro/Nanofiber Properties Density, g/cm³ 1.17 TensileStrength, MPa 450 Modulus, GPa 6.0 Elongation, % 15 Typical FiberLength, mm 4.5-6.5 Canadian Standard Freeness, ml  10-700 BET SurfaceArea, m²/g 50 Moisture Regain, % <2.0 Surface Charge Anionic

Such fibers are actually present, as discussed above, in a pulp-likeformulation, thereby facilitating introduction within a wetlaid nonwovenfabric production scheme.

Nonwoven Production Method

Material combinations were then measured out to provide differingconcentrations of both components prior to introduction together into awet-laid manufacturing process. Handsheets were made according to TAPPITest Method T-205, which is incorporated here by reference (basically,as described above, mixing together in a very high aqueous solventconcentration formulation and under high shear conditions as aretypically used in wet laid manufacturing and described as “refining” offibers, ultimately laying the wet structure on a flat surface to allowfor solvent evaporation). Several different combinations were producedto form final nonwoven fabric structures. The method was adjusted onlyto accommodate different basis weights by adjusting the initial amountof material incorporated into each sheet. Materials and ratios are shownin Table 2.

FIGS. 5 and 6 correlate in structure to Example 39, below, as well. Thesimilarity in structure (larger microfibers and smaller nanofibers) areclarified, and the presence of fewer amounts of nanofibers in thesestructures is evident from these photomicrographs, as well.

The fabric was measured for thickness and then cut into suitable sizesand shapes for introduction within lithium ion rechargeable batterycells. Prior to any such introduction, however, samples of the batteryseparator fabrics were analyzed and tested for various properties inrelation to their capability as suitable battery separators.Furthermore, comparative examples of battery separator nanofibermembranes according to U.S. Pat. No. 7,112,389, which is herebyincorporated by reference, as well as battery separator films fromCelgard, are reported from the tests in the patent and from Celgardproduct literature.

EXAMPLES

Examples 36-51 were made according to TAPPI Test Method T-205 usingEngineered Fiber Technologies EFTEC™ A-010-04 fibrillated acrylic fiber(combination of microfiber and nanofiber)(listed as Base Fiber) andFiberVisions T426 fiber, which is 2 denier per filament, cut to 5 mmlength, a bicomponent fiber made from polypropylene and polyethylene,and has a diameter of approximately 17 microns (listed as Added Fiber).The sheets were calendered between two hard steel rolls at 2200pounds/linear inch at room temperature (˜25 C). The amount of eachfiber, conditioned basis weight, caliper (or thickness), apparentdensity and porosity of the examples are shown in Table 4. ConditionedBasis Weight, Caliper, Apparent Density, and Tensile were testedaccording to TAPPI T220, which is hereby incorporated by reference.

TABLE 2 Separator Properties % % Conditioned Apparent Base Added BasisWt Caliper Density Porosity Example Fiber Fiber g/m² mm g/cm³ % 36 100 039.9 0.065 0.614 56.2% 37 90 10 40.2 0.067 0.600 55.6% 38 80 20 39.80.068 0.585 55.0% 39 70 30 39.9 0.07 0.570 54.4% 40 100 0 29.98 0.0510.588 58.0% 41 90 10 29.89 0.053 0.564 58.2% 42 80 20 28.91 0.054 0.53558.8% 43 70 30 30.9 0.074 0.418 66.6% 44 100 0 23.58 0.044 0.536 61.7%45 90 10 24.8 0.046 0.539 60.1% 46 80 20 24.75 0.047 0.527 59.5% 47 7030 24.15 0.053 0.456 63.5% 48 100 0 14.8 0.03 0.493 64.8% 49 90 10 16.60.036 0.461 65.8% 50 80 20 16.4 0.033 0.497 61.8% 51 70 30 16.5 0.0370.446 64.3%

The higher the porosity, the higher the peak power output within thesubject battery. With such high results, theoretically, at least, thenumber of batteries necessary to accord the necessary power levels torun certain devices (such as hybrid automobiles, for instance) would bereduced through an increase in the available power from individualbatteries. Such a benefit would be compounded with an effective airresistance barrier as well. The porosity of the inventive separator mayalso be controlled by the ratio of nanofiber to microfibers, the typesof nanofibers, and also by post processing such as calendaring, as canbe seen below.

Battery Separator Base Analysis and Testing

The test protocols were as follows:

Porosity was calculated according to the method in U.S. Pat. No.7,112,389, which is hereby incorporated by reference. Results arereported in %, which related to the portion of the bulk of the separatorthat is filled with air or non-solid materials, such as electrolyte whenin a battery.

Gurley Air Resistance was tested according to TAPPI Test Method T460,which is hereby incorporated by reference. The instrument used for thistest is a Gurley Densometer Model 4110. To run the test, a sample isinserted and fixed within the densometer. The cylinder gradient israised to the 100 cc (100 ml) line and then allowed to drop under itsown weight. The time (in seconds) it takes for 100 cc of air to passthrough the sample is recorded. Results are reported in seconds/100 cc,which is the time required for 100 cubic centimeters of air to passthrough the separator.

Mean Flow Pore Size was tested according to ASTM E-1294 “Standard TestMethod for Pore Size Characteristics of Membrane Filters Using AutomatedLiquid Porosimeter” which uses an automated bubble point method fromASTM F 316 using a capillary flow porosimeter. Tests were performed byPorous Materials, Inc., Ithaca, N.Y.

The air permeability of a separator is a measurement of the timerequired for a fixed volume of air to flow through a standard area underlight pressure. The procedure is described in ASTM D-726-58.

TABLE 3 Tensile properties and Mean Flow Pore Size Mean Flow MD TensileCD Tensile Pore Size Example kg/cm² kg/cm² microns 36 94 94 0.13 37 8585 0.13 38 67 67 0.15 39 59 59 0.20 40 88 88 0.15 41 69 69 0.18 42 51 510.25 43 29 29 0.62 44 74 74 0.19 45 65 65 0.23 46 56 56 0.27 47 40 400.69 48 52 52 49 57 57 50 42 42 51 34 34

The inventive example thus shows a very small pore size mean, indicatinga capability to permit a large number of recharge cycles for the subjectbattery. In addition, the ability to control the pore size is indicatedby the change in pore size with the proportional change in the ratio ofnanofiber and microfiber materials. This is a key advantage that is notpresent in any previous art, such that with this technology the poresize can be dialed in by the battery manufacturer depending on therequirements of the end user. Thus, a separator can be designed for apower tool or automotive application to have different characteristicsfrom a rechargeable watch battery, cell phone or laptop computer.

The tensile properties in the examples given are isotropic, that is, thesame in all directions, with no distinction between machine and crossdirections. Comparative examples show tensile properties that varyconsiderably between machine direction (MD) and cross direction (CD)tensile strength. In general, nanofiber-based battery separators arequite weak. Thus, one advantage of the current invention is the tensilestrength, which allows faster processing in battery manufacture, tighterwinding of the batteries, and more durability in battery use. Such MDtensile strength is preferably greater than 25 kg/cm², more preferablygreater than 50 kg/cm², and most preferably greater than 100 kg/cm². Therequirements on the CD tensile strength are lower, preferably beinggreater than 10 kg/cm², more preferably being greater than 25 kg/cm²,and most preferably greater than 50 kg/cm².

As noted above, calendering and an increased population of nanofibersrelative to microfibers will reduce the overall pore size mean, evenfurther, thus indicating, again, the ability to target certainmeasurements on demand for the inventive technology. Sheet production ofthe initial separator was then undertaken on a paper making machine (toshow manufacturing may be simplified in such a manner) with such acalendering, etc., step undertaken as well.

Paper Machine Production

Two materials were then made on a rotoformer paper machine. The first,Example 52, was made from 75% EFTec A-010-4 and 25% 0.5 denier/filamentpolyethylene terephthalate (PET) fiber with cut length 6 mm. The second,Example 53, was made from 37.5% EFTec A-010-4, 37.5% EFTec L-010-4 and25% PET fiber with cut length 6 mm. The fiber materials were dispersedusing high shear mixing and mixed at high dilution in water, then fedinto the rotoformer head box and made to sheets of weight 20 grams/m²and dried in a hot air oven. The resultant rolls were calendered at 325°F. at 2200 pounds/linear inch, resulting in thicknesses of ˜40 micronsfor the first sheet and 30 microns for the second sheet. Shrinkage wasmeasured at 90° C., 130° C., and 160° C. by measuring a 12″ length ineach of machine and cross direction, placing in an oven stabilized atthe measuring temperature for 1 hour, and measuring the length again.The shrinkage is the change in length expressed as a percentage of theoriginal length. Properties of the sheets are shown below in Table 4.

TABLE 4 Membrane Properties Unit of Exam- Exam- Basic Membrane PropertyMeasure ple 52 ple 53 Thickness μm 40 30 Gurley (JIS) seconds 20 110Porosity % 60% 55% Mean Flow Pore Size μm 0.5 0.5 TD Shrinkage @ 90 C./1Hour % 0 0 MD Shrinkage @ 90 C./1 Hour % 0 0 TD Shrinkage @ 130 C./1Hour % 0 0 MD Shrinkage @ 130 C./1 Hour % 2 1 TD Shrinkage @ 160 C./1Hour % 1 0 MD Shrinkage @ 160 C./1 Hour % 4 2 TD Shrinkage @ 190 C./1Hour % 5 0 MD Shrinkage @ 190 C./1 Hour % 7 2 TD Strength Kg/cm² 70 100MD Strength Kg/cm² 190 170 Elongation %  4%  4%

As can be seen, the materials with both acrylic (EFTec A-010-4) andlyocell (EFTec L-010-4) materials show very good properties at hightemperature. For example, many current stretched film separators may bemade from polyethylene, which melts at 135° C. and shows significantshrinkage at over 110° C., or from polypropylene, which melts at 160° C.and shows significant shrinkage over 130° C. One problem that is knownin the industry, especially for large format cells that might be used inelectric vehicles, is that shrinkage upon exposure to high temperaturecan expose the electrodes to touching each other on the edges if theseparator shrinks, causing a short and potentially a catastrophicthermal runaway leading to an explosion. Separators with hightemperature stability thus are safer in these environments, allowinglarger format cells to be used with higher energy per cell. Preferredseparator performance might be to have less than 10% shrinkage at 130°C., 160° C. or 190° C. in both directions, or preferably less than 6%shrinkage or most preferably less than 3% shrinkage. In addition, theseparator might be made with a component that has high temperaturestability such as a lyocell, rayon, para-aramid, meta-aramid, or otherfiber, that when formed into a sheet with other materials imparts a lowshrinkage result, as is shown in Example 53.

Additional examples were made and tested for different calenderingconditions. The paper was constructed on a Rotoformer at the HeftyFoundation facility, and consisted of 27% EFTec A-010-04 acrylicnanofiber, 53% EFTec L-010-04 lyocell nanofiber, and 20% 0.5denier/filament polyester fiber with 5 mm cut length. The materials weremixed for 40 minutes in a 1000 gallon hydropulper, and then fed into themachine at approximately 0.25% fiber content, and a sheet made that was15 grams/m² in areal density. This paper was calendered under differentconditions, which are listed below and shown as Examples 56-60 in theTable 5 below.

Legend for Examples 56-60:

56: Calendered using the conditions above, except the rolls were notheated.

57: Sheet was fed through the calender with a second sheet of Example56, plying the sheets together.

58: Sheet from 56 was fed through the calender with a roll of copy paper(need wt??), then peeled from the copy paper.

59: Sheet from 56 was calendered with a second pass under the sameconditions.

60: The plies of 57 were peeled apart, resulting in two separate sheets.

Two things can be seen from the examples below. First, the lamination oftwo sheets gives more than twice the Gurley air resistance of a singlesheet, while lowering the total porosity. Second, calendaring a secondtime had the effect of increasing the porosity and lowering the Gurley.Last, the two sheets that were fed through with another sheet had theeffect of increasing the Gurley and increasing the porosity at the sametime. Tensile strength was decreased in all cases with additionalcalendering.

TABLE 5 Calendered Sheet Results Gurley Conditioned Apparent MD CD AirBasis Wt Caliper Density Porosity Tensile Tensile Resistance Exampleg/m² mm g/cm³ % kg/cm² kg/cm² seconds 56 14.7 0.031 0.474 59.6% 155 6938 57 30.0 0.060 0.500 57.4% 136 53 105 58 15.2 0.037 0.412 64.9% 102 4448 59 15.1 0.036 0.419 64.2% 99 40 34 60 15.0 0.036 0.415 64.6% 94 43 40Wettability Testing

A square of Example 39 was taken along with a square of Celgard 2320,and a drop of 1 M LiPF6 in EC:DMC:DEC mixture (1:1:1 by volume)electrolyte was placed on the surface. After 5 seconds, the electrolytehad been completely absorbed into Example 39, with no spectralreflectance (i.e., differing spectral measurements at differing angleswith such differences generated from the shiny surface of a liquid dropformation on a surface) observable. To the contrary, the electrolytedrop on the Celgard 2320 remained far in excess of 5 seconds withoutfull wicking throughout the structure. This spectral reflectance resultfor the inventive material is highly desirable for a lithium ion batteryseparator to increase the processing rate of dispersing the electrolyte,as well as to ensure uniform dispersion of the electrolyte on and withinthe separator itself. Non-uniform dispersion of the electrolyte is knownto promote dendrite formation on repeated charge and discharge, whichbecome defects in the cells and can lead to short circuits.

As such, it may be desirable to have a separator exhibiting a uniformspectral reflectance on its surface after 5 minutes of liquidelectrolyte deposition (in drop form), preferably less than 2 minutesduration, and more preferably less than 1 minute duration. In addition,it may be desirable to make an energy storage device from twoelectrodes, a separator and an electrolyte, such that the separatorexhibits the same spectral reflectance measurements in the same manner.

As it is, the inventive separator exhibited such a spectral reflectancemeasurement of at most 5 seconds in each instance (most tests showed 2seconds and less for such a result), exhibiting effective wicking (andthus uniform dispersion) of the liquid electrolyte throughout theentirety of the separator.

Other tests were undertaken involving Differential Scanning calorimetryand Thermogravimetric Analysis for Wettability measurements as well.Example 53 was tested for thermogravimetric analysis from roomtemperature to 1000° C. The sample showed 1.39% mass loss, ending near100° C., which is consistent with water loss from the cellulosenanofibers and microfibers. The material showed no further degradationuntil approximately 300° C., when oxidation set in and a sharp decreaseof approximately 60% mass between 335 and 400° C. The Example 53 wasalso tested for differential scanning calorimetry from room temperatureto 300° C. There was a broad exotherm centered around 100° C.,consistent with a release of water, and a sharper exotherm at 266° C.which onset at 250° C., consistent with the melting point of PET.

Example 52 was tested for thermogravimetric analysis from roomtemperature to 1000° C. The sample showed very little mass loss below300° C., with an onset of mass loss at 335° C., and an approximately 40%mass loss up to 400° C. The Example 52 was also tested for differentialscanning calorimetry from room temperature to 300° C. There was almostno signature shown between room temperature and a sharp exotherm at 266°C., onset at 250° C., consistent with the melting point of PET. Inshort, the curve showed no signature other than the melting of the PETmicrofibers.

Battery Formation and Actual Battery Testing Results

FIG. 7 shows the typical battery 10 structure with the outside housing12 which includes al of the other components and being securely sealedto prevent environmental contamination into the cell as well as anyleakage of electrolyte from the cell. An anode 14 is thus supplied intandem with a cathode 16, with at least one battery separator 18 betweenthe two. An electrolyte 20 is added to the cell prior to sealing toprovide the necessary ion generation. The separator 18 thus aids inpreventing contact of the anode 14 and cathode 16, as well as to allowfor selected ion migration from the electrolyte 20 therethrough. Thegeneral format of a battery cell follows this structural description,albeit with differing structures sizes and configurations for eachinternal component, depending on the size and structure of the batterycell itself. In this situation, button battery of substantially circularsolid components were produced for proper testing of separatoreffectiveness within such a cell.

To that end, electrical properties of the separator were tested first bymaking symmetric lithium foil-separator-lithium foil 2016 coin cells andtesting for electrical resistance, and then by making asymmetric carbonelectrode-separator-lithium foil 2016 coin cells. Testing was done atthe Nanotechnology Laboratory in the Georgia Institute of TechnologySchool of Materials Science and Engineering. For the symmetriclithium—separator—lithium 2016 coin cells, ⅝″ rounds were cut fromselected separators, dried in a vacuum chamber of an Ar-filled glove boxat 70° C. for approximately 12 hours and assembled into:

(a) symmetric lithium foil-separator-lithium foil 2016 coin cells and

(b) asymmetric carbon electrode-separator-lithium foil 2016 coin cells.

The electrolyte used was 1 M LiPF6 in EC:DMC:DEC mixture (1:1:1 byvolume). Lithium foil was rolled to thickness 0.45 mm and one or twolayers of separator were used in this study. A Celgard 2325 separatorwas used for comparison test purposes as well.

After 2 days of storage, the potentiostatic electrochemical impedancespectroscopy (EIS) measurements in the frequency range from 0.01 Hz to100 kHz were carried out on each of the assembled two electrodeLi-separator-Li coin cells.

Each cell included the following contributors to the total resistance:(i) Li ion transport in the electrolyte/separator; (ii) Li ion transportin a solid-electrolyte-interphase (SEI) layer on each of the Lielectrodes; (iii) electron transport in Li/cell/contacts. Among thesecomponents of the resistance the (iii) electron transport can generallybe neglected, while (i) Li ion transport in electrolyte usually give nosemicircle in the present frequency region due to their highcharacteristic frequencies.

Being primarily interested in (i) Li ion transport in theelectrolyte/separator, attention was centered on the high frequencyregion of the Nyquist plot associated therewith. The total resistance ofthe ion transport across the separator was approximated as the value ofthe Real part of the total resistance Z at high frequency where theimaginary component of the complex impedance becomes zero. As previouslymentioned, the electrical resistance of the interfaces and theelectrodes is much smaller than the ionic resistance and thus could beneglected.

Further Battery Products and Tests

Additional pouch cell batteries were built as follows: Standard cellphone battery electrodes have a coat weight that is approximately 2.5mAh/cm². Electrodes were produced for test procedures exhibiting a coatweight of 4 mAh/cm² (NCA) to demonstrate that the capability limits ofthe separator were exceeded versus standard practices as it pertained torate capability. One cell (hand built) of each separator type was builtwith Celgard 2325 (Example 54, below) and Example 53 (Example 55,below). The electrodes were coated, calendered, dried, welded with tabs,put into laminate packaging, and filled with a 1M Li salt in a standardbattery solvent electrolyte, and sealed. The cells were tested fordischarge capacity at C/10, C/4, C/2 and C rates with several dischargesat each rate, and the results are shown in Table 7 below as a percentageof the first discharge at C/10 capacity after formation. The specificdischarge capacity at C/10 for the Example 54 cell was 141 mAh/g and forExample 55 cell was 145 mAh/g.

TABLE 6 Pouch Battery Measurements Rate Example 54 Example 55 C/10100.3% 101.3% C/4 95.5% 98.3% C/2 69.5% 88.7% C 36.4% 57.1%

As can be seen from these examples, the battery made using the inventiveseparator had higher discharge capacity at higher rates, with a smalladvantage at C/4, but larger and significant advantages at rates of C/2and C.

It should be understood that various modifications within the scope ofthis invention can be made by one of ordinary skill in the art withoutdeparting from the spirit thereof. It is therefore wished that thisinvention be defined by the scope of the appended claims as broadly asthe prior art will permit, and in view of the specification if need be.

I claim:
 1. A method of forming a single layer battery separator,wherein said battery separator exhibits a maximum thickness of 250microns, and wherein said battery separator includes a combination ofmicrofiber and nanofiber constituents, said method comprising the stepsof: a) providing an aqueous solvent; b) introducing therein a pluralityof nanofibers having an average width of less than 700 nm to form ananofiber dispersion within an aqueous solvent; c) mixing said nanofiberdispersion under high shear conditions of at least 14.13 m/sec linearspeed; d) introducing a plurality of microfibers having an average widthof greater than 3000 nm to form a microfiber/nanofiber dispersion withinsaid aqueous solvent such that said microfiber/nanofiber dispersion hasa concentration of fibers solids of less than 1.0% by weight of solvent;e) introducing said dispersion of step “d” within a paper makingmachine; f) producing a web of microfiber/nanofiber material; and g)drying said web.
 2. The method of claim 1 wherein said resultant web ofstep “f” is further treated in a calendering procedure to produce aseparator material exhibiting a thickness of at most 100 microns and apore size of at most 2000 nm.
 3. The method of claim 1 wherein saidmicrofiber is fibrillated.
 4. The method of claim 1 wherein saidmicrofiber is an island-in-the-sea microfiber.
 5. The method of claim 1wherein said nanofiber is fibrillated.
 6. The method of claim 1 whereinsaid nanofiber is an island-in-the-sea nanofiber.
 7. The method of claim1 wherein said microfiber is of a length of at least 1 mm.
 8. The methodof claim 1 wherein said microfiber/nanofiber dispersion has aconcentration of fibers solids of less than 0.5% by weight of water. 9.The method of claim 1 wherein said paper making machine is chosen fromthe group consisting of rotoformer, inclined wire, and Fourdriniermachines.
 10. The method of claim 2 wherein said battery separator hasan areal density of less than 30 grams/m².
 11. The method of claim 1wherein said battery separator has an areal density of less than 20grams/m².
 12. The method of claim 1 further comprising the step of usinga vacuum to reduce the water content of the sheet.
 13. The method ofclaim 1 in which the paper making machine comprises a woven papermachine wire for holding the fibers while removing water, said wirehaving a gauge finer than 40 gauge.
 14. The method of claim 2 in whichthe calendar is heated to a temperature greater than 200° F.
 15. Themethod of claim 1 in which the average diameter of the microfibers isgreater than 10 times the average diameter of the nanofibers.
 16. Themethod of claim 15 in which the average diameter of the microfibers isgreater than 10 times the average diameter of the nanofibers.
 17. Themethod of claim 1 such that the separator has a thermal shrinkage in240° C. for one hour of less than 6% in both the machine direction andthe cross direction.
 18. The method of claim 1 such that the thicknessof said single layer separator is less than 100 microns.
 19. An energystorage device formed from two electrodes, a separator made inaccordance with the method of claim 1, and an electrolyte.
 20. Themethod of claim 1 comprising at least one fiber that will flow underhigh temperature and/or pressure and at least one fiber that will notflow under the same temperature and/or pressure.
 21. The method of claim1 comprising a microfiber with length greater than 0.5 mm.